1. Field of Invention
The present invention generally relates to apparatus and method for dispensing a process liquid characterized by a high viscosity and a short shelf life.
2. Related Art
Semiconductor manufacturing processes frequently employ process liquids of high viscosity, such as polyimides (typically having a viscosity of 250–35,000 centipoises), which exhibit a good combination of thermal stability, mechanical toughness and chemical resistance and can be used as protective overcoats, interlayer dielectrics, or passivation layers in microelectronic applications. Due to their high viscosity, these process liquids are usually dispensed from pressurized storage and dispensing vessels, by special dispensing pumps in conjunction with large diameter tubing (e.g., 0.9525 cm, 0.375 inch diameter).
A recirculation loop downstream of the dispensing pump is usually provided to keep the high viscosity process liquid in continuous fluidic motion at a desired flow rate. Such a recirculation loop reduces solidification of the liquid (e.g., gel slug formation) inside the dispensing lines, prolongs the shelf life of such liquid, and provides a means for purging air out of the dispensing lines. The recirculation loop usually comprises a three-way dispensing/recirculating valve, a recirculating line, and a recirculating probe coupled to the fluid vessel, for re-circulating the high-viscosity process liquid back into such vessel.
Conventional recirculation probes comprise an output flow path connected to an output port, and a return flow path connected to a recirculating port. A process liquid flows out of the fluid vessel via the output flow path and the output port, and re-circulated process liquid flows back into the fluid vessel via the recirculating port and the return flow path. Typically, the cross-sectional flow area of the return flow path is much smaller than that of the output flow path. Therefore, when the output liquid volume is substantially equal to the re-circulated liquid volume (as usually occurs when purging gas out of the dispensing lines), such difference in cross-sectional flow areas of the output and return flow paths causes an imbalance of discharge pressures in the dispensing line and in the recirculating line. This imbalance unduly burdens the dispensing pump and the dispensing/recirculating valve and causes the pump and the valve to wear out prematurely.
Moreover, conventional recirculation probes feature separate tubing for the output flow path and the return flow path. Such separate tubing configuration does not effectively use the limited cross-sectional area of the opening of the fluid storage and dispensing vessel.
Further, the return flow path of conventional re-circulation probes terminates right below the neck portion of the fluid vessel, in order to minimize the inner surface area of the return flow path and to reduce the head losses caused by the flow resistance of the inner surface of the return flow path. However, such design leaves a free space between the end of the return flow path and the liquid surface within the fluid vessel, and the re-circulated liquid therefore drips in a free-fall manner from the return flow path into the fluid vessel, causing liquid turbulence and deleterious formation of air bubbles in the fluid vessel.
It is therefore one object of the present invention to reduce or eliminate the pressure imbalance between the dispensing line and the recirculating line, so as to prolong the useful life of the dispensing pump and the dispensing/recirculating valve.
It is another object of the present invention to effectively use the limited cross-sectional area of the opening of the fluid storage and dispensing vessel, and to concurrently maximize the effective flow area of the output and return flow paths.
It is still another object of the present invention to provide a smooth flow of the re-circulated fluid back into the fluid storage and dispensing vessel, so as to reduce liquid turbulence and formation of air bubbles in such vessel, without significantly increasing the inner surface area of the return flow path.
It is a still further object of the present invention to provide a liquid recirculating system with changeable liquid outflow ports and/or recirculation ports, and to enable sealed dispensing of high-viscosity liquids that eliminates exposure of such liquids to airborne contaminates and eliminates exposure of personnel to the hazardous fumes of such liquids.
Other objects and advantages will be more fully apparent from the ensuing disclosure and appended claims.